Power turbine speed control using electrical load following

ABSTRACT

A power turbine speed control system for a turbo-shaft type gas turbine engine that has a gas generator compressor spool and a power turbine spool and drives an electrical generator that powers at least one electrical load by way of at least one electrical bus, comprises a power turbine controller that senses the rotary speed of the power turbine spool and generates at least one signal that changes the torque of the electrical generator in response to the sensed change in the rotary speed of the power turbine spool.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Divisional Patent Application for the patent application Ser.No. 12/568,439 and claims the benefit of the priority date there for.Patent application Ser. No. 12/568,439 is a Divisional PatentApplication for the patent application Ser. No. 11/642,401 filed 20 Dec.2008, now U.S. Pat. No. 7,615,881.

FIELD OF THE INVENTION

The invention relates to a speed control for a turbo-shaft type gasturbine engine that drives an electrical power generation system, andmore particularly to a speed control for a gas turbine engine drivenelectrical power generation system that maintains a substantiallyconstant engine speed despite changes in electrical loading.

BACKGROUND OF THE INVENTION

As aeronautical gas turbine engines of the free turbine turbo-shaft typefind expanding applications for power generation applications due totheir high achievable power densities, it is becoming increasingimportant to obtain the maximum transient response capability from theengines. In particular, many of these power generation applications arestriving to obtain the most efficient and power dense power generationpossible. This efficiency may rely on superconducting generators thathave much longer excitation system time constants than the gas turbineengine. Although the two to four second time constant of the aircraftgas turbine engine will never satisfy the near instantaneous load-on andload-off transient capability of electrical systems, obtaining themaximum transient capability from the engine will greatly reduce thetransient handling requirements for any associated energy storage andmanagement systems, such as capacitors, batteries, flywheels and soforth. Furthermore, even if a conventional synchronous generator thatcan obtain very fast load-on and load-off power transients providespower generation, it may not be desirable to utilise this capabilitybecause of the transient response limitations of the engine. Ideally,the electrical load should follow the transient response of the gasturbine to maintain power turbine speed control.

SUMMARY OF THE INVENTION

The invention generally comprises a power turbine speed control systemfor a turbo-shaft type gas turbine engine that has a gas generatorcompressor spool and a power turbine spool and drives an electricalgenerator that powers at least one electrical load by way of at leastone electrical bus, comprising a power turbine controller that sensesthe rotary speed of the power turbine spool and generates at least onesignal that changes the torque of the electrical generator in responseto the sensed change in the rotary speed of the power turbine spool.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a power turbine spool speed controlsystem a turboshaft type gas turbine engine according to a firstpossible embodiment of the invention.

FIG. 2 is a schematic diagram of a power turbine speed control systemfor a turboshaft type gas turbine engine according to a second possibleembodiment of the invention.

FIG. 3 is a schematic diagram of a power turbine speed control systemfor a turboshaft type gas turbine engine according to a third possibleembodiment of the invention.

FIG. 4 is a diagram of compressor spool speed/power turbine spool torqueand controlled generator power output as a function of time.

FIG. 5 shows a diagram of compressor spool speed/power turbine spooltorque and controlled generator power output as a function of timeimproved with both storage and parasitic load approaches to control ofan energy management system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of a power turbine speed control system 2for a turboshaft type gas turbine engine 4 according to a first possibleembodiment of the invention. The gas turbine engine 4 comprises a gasgenerator compressor spool 6 and a power turbine spool 8. The gasgenerator compressor spool 6 comprises a gas generator turbine 10 thatdrives a gas generator compressor 12 by way of a gas generator shaft 14.The power turbine spool 8 comprises a power turbine 16 that drives apower turbine shaft 18.

The gas turbine engine 4 drives an electrical generator 20 by way of thepower turbine shaft 18. Although the gas generator spool 6 generatescombustion gas that imparts torque to the power turbine spool 8, therotational speed of the power turbine spool 8, and thus rotational speedof the power turbine shaft 18 and the electrical generator 20, ismechanically independent of the rotational speed of the gas generatorspool 6.

The electrical generator 20 is generally a polyphase alternating current(AC) generator, typically of the three-phase type, that generates poweron an AC bus 22. The AC power on the AC bus 22 may drive AC electricalloads directly, may drive direct current (DC) loads through appropriaterectification, or both. When AC loads alone are present, such asrepresented by AC loads 24, the AC loads 24 receive the generated powerdirectly from the AC bus 22. When DC loads are present, a rectifiersystem 24 receives AC electrical power from the AC bus lines 22 andconverts it to direct current (DC). The rectifier system 24 may be ofthe passive or active rectifier type.

The gas turbine engine 4 develops a power turbine spool rotary speedsignal. A power turbine speed controller 26 receives the power turbinespool rotary speed signal from the gas turbine engine 4 by way of apower turbine rotary speed signal line 28. The power turbine controller26 senses the rotary speed of the power turbine spool 8 from the levelof the power turbine spool rotary speed signal. The power turbinecontroller 26 produces an excitation signal for the generator 20 on anexcitation signal line 30 that changes in response to speed variationsof the power turbine shaft 18. These variations may result from anychange in input torque supplied by the gas generator compressor spool 6or load torque exerted by the generator 20 due to increased power drawon the AC bus 22.

For instance, if the power turbine controller 26 senses a drop in rotaryspeed of the power turbine spool 8 under a predetermined operatingspeed, the power turbine controller 26 reduces the level of theexcitation signal that the generator 20 receives on the excitation line30 so that it reduces its output and therefore the torque that it exertson the power turbine spool 8, thereby letting the rotary speed of thepower turbine spool 8 return to its predetermined operating speed. Ifthe power turbine controller 26 senses an increase in rotary speed ofthe power turbine spool 8 over the predetermined operating speed, thepower turbine controller 26 increases the level of the excitation signalthat the generator 20 receives on the excitation line 30 so that itincreases its output and therefore the torque that it exerts on thepower turbine spool 8, thereby letting the rotary speed of the powerturbine spool 8 return to its predetermined operating speed.

The power turbine speed control system 2 may also comprise a load powersensor 32 that senses the power consumed by the AC loads 24. The loadpower sensor 32 generates a load power signal representative of thepower consumed by the AC loads 24 on a load power signal line 34. Thepower turbine speed control system 2 may further comprises a fuel systemcontroller 36 that receives the load power signal on the load powersignal line 34.

The power turbine speed control system 2 may still further comprise anenergy management system 38, such as a flywheel, battery or capacitorback fed by a bi-directional inverter/rectifier, that stores energy thatit may feed back into the AC bus 22 by way of storage supply line 40.The energy management system 38 generates an energy storage levelfeedback signal on a storage signal line 42. The fuel system controllermay receive the storage signal on the storage signal line 42 and compareit to the load power signal on the load power signal line 34 to generatea fuel flow command signal on a fuel command line 44. The gas turbineengine 4 receives the fuel flow command signal on the fuel command line44 and adjusts its fuel flow accordingly to compensate for any increaseor decrease in the consumption of power on the AC bus 22 by the AC loads24 as well as the level of stored energy in the energy management system38. If transient changes in power by the loads 24 are predictable, thefuel system controller 36 may respond to a feed forward fuel command ona feed forward signal line 46 and adjust the fuel flow command signal onthe fuel command line 44 accordingly.

As hereinbefore stated, the AC power on the AC bus 22 may drive ACelectrical loads directly, may drive DC loads through appropriaterectification, or both. FIG. 2 is a schematic diagram of a power turbinespeed control system 48 for a turboshaft type gas turbine engine 4according to a second possible embodiment of the invention that isuseful when DC loads are present, such as represented by DC loads 50. Apassive rectifier system 52 receives AC electrical power from the AC bus22 and converts it to DC power on a DC bus 54. In this case, the energymanagement system 38 may comprise various DC energy managementsolutions, such as a capacitor bank, a battery or a bi-directionalinverter/rectifier fed flywheel. The energy that it stores energy may inthis case feed back into the DC bus 54 by way of the storage supply line40. This embodiment otherwise operates in the same manner as the powerturbine speed control system 2 hereinbefore described in connection withFIG. 1.

FIG. 3 is a schematic diagram of a power turbine speed control system 56for a turboshaft type gas turbine engine 4 according to a third possibleembodiment of the invention that is also useful when DC loads arepresent, such as represented by the DC loads 50. It uses an activerectifier system 58 instead of the passive rectifier system 52 thatcomprises the power turbine speed control system 48 hereinbeforedescribed in connection with FIG. 2. The active rectifier 58 may be astandard six-switch IGBT (MOSFET or other switching device) rectifieremploying IGBTs, MOSFETs or other switching devices, a multi-levelrectifier, or a phase controlled SCR based rectifier. In thisembodiment, the power turbine controller 28 produces gate drive signalsfor the active rectifier system 58 on a gate drive signal line 60 thatchanges in response to speed variations of the power turbine shaft 18.These variations may result from any change in input torque supplied bythe gas generator compressor spool 6 or load torque exerted by thegenerator 20 due to increased power draw on the AC bus 22.

For instance, if the power turbine controller 26 senses a drop in rotaryspeed of the power turbine spool 8 under a predetermined operatingspeed, the power turbine controller 26 varies the gate drive signalpulses that the active rectifier system 58 receives on the gate drivesignal line 60 so that it reduces its output and therefore the load thatit exerts on the generator 20 and in turn the torque that the generator20 exerts on the power turbine spool 8, thereby letting the rotary speedof the power turbine spool 8 return to its predetermined operatingspeed. If the power turbine controller 26 senses an increase in rotaryspeed of the power turbine spool 8 over the predetermined operatingspeed, the power turbine controller 26 varies the gate drive signalpulses that the active rectifier system 58 receives on the gate drivesignal line 60 so that it reduces its output and therefore the load thatit exerts on the generator 20 and in turn the torque that the generator20 exerts on the power turbine spool 8, thereby letting the rotary speedof the power turbine spool 8 return to its predetermined operatingspeed.

Of course, where a combination of AC and DC loads are present, a powerturbine speed control system according to the invention may comprise anysuitable combination of the first, second and third embodiments of theinvention hereinbefore described in connection with FIGS. 1, 2 and 3. Inother words, a power turbine control system according to the inventionmay have a combination of AC loads 24 supplied by AC bus 22 and DC loads50 supplied by DC bus 54, either the active rectifier system 58, thepassive rectifier system 52 or their combination, and the power turbinecontroller 28 may generate an excitation signal for the generator 20, agate drive signal for the active rectifier system 48 and or theircombination, as shall be appreciated by those skilled in the art.

FIG. 4 is a diagram of compressor spool speed/power turbine spool torqueand controlled generator power output as a function of time for thethree possible embodiments of the invention hereinbefore described inconnection with FIGS. 1 through 3. Line 60 represents rotary speed ofthe power turbine spool 8 as a function of time. Line 62 representsrotary speed of the gas generator compressor spool 6 as a function oftime, and therefore torque input to the power turbine spool 8 as afunction of time. Line 64 represents power output of the generator 20 asa function of time, and therefore torque exerted on the power turbinespool 8 as a function of time. Line 66 represents the transient responsetime of the gas generator compressor spool 6 as a function of time,typically in the range of approximately 2 to 3 seconds. The rotary speedof the power turbine spool 8 remains constant regardless of thetransient response time of the gas generator compressor spool 6 becausethe power turbine controller 26 forces the output of the generator 20,and thus the torque that it exerts on the power turbine spool 8, totrack the available torque from the generator compressor spool 6, evenduring its transient periods.

It is possible to improve the transient performance of the threepossible embodiments of the invention hereinbefore described inconnection with FIGS. 1 through 3 further by suitable control of theenergy management system 38. FIG. 5 shows both storage, such as battery,flywheel, capacitor bank, and so forth, and parasitic load approaches tosuch control of the energy management system 38. Line 68 represents thedesired transient response of the three possible embodiments of theinvention.

For the storage approach, line 70 represents energy that the energymanagement system 38 adds to or subtracts from the load current on itsrespective bus as a function of time. Regions 72 represent the energymanagement system 38 acting as an energy sink and regions 74 representthe energy management system 38 acting as an energy source. Line 68 thenrepresents the combined response of line 64 representing power output ofthe generator 20 and line 70 representing energy that the energymanagement system 38 adds to or subtracts from the rapid transitioningload power on its respective bus as a function of time, resulting in analmost transition free change in generated electrical power.

For the parasitic load approach, line 76 represents the amount of energythat the energy management system 38 dissipates as a function of time.Line 68 then represents the combined response of line 64 representingthe rapid transient load and line 76 representing the amount of energythat the energy management system 38 dissipates as a function of timesimilarly results in an almost transition free change in generatedelectrical power.

The described embodiments of the invention are only some illustrativeimplementations of the invention wherein changes and substitutions ofthe various parts and arrangement thereof are within the scope of theinvention as set forth in the attached claims.

1. A power turbine speed control system for a turbo-shaft type gasturbine engine that has a gas generator compressor spool and a powerturbine spool and drives an electrical generator that powers at leastone electrical load by way of at least one electrical bus, comprising: apower turbine controller that senses a rotary speed change of the powerturbine spool and generates at least one signal that changes a torque ofthe electrical generator in response to the sensed change in the rotaryspeed of the power turbine spool, at least one alternating current (AC)bus powered by the electrical generator; and at least one AC loadpowered by the AC bus; wherein the power turbine controller generates agenerator excitation signal that changes the torque of the electricalgenerator.
 2. The power turbine speed control system of claim 1, furthercomprising: at least one AC bus powered by the electrical generator; atleast one direct current (DC) bus; at least one DC load powered by theDC bus; and at least one AC to DC rectifier system that converts poweron the AC bus to DC power for the DC bus.
 3. The power turbine speedcontrol system of claim 2, wherein the rectifier system is of thepassive type and the power turbine controller generates a generatorexcitation signal that changes the torque of the electrical generator.4. The power turbine speed control system of claim 2, wherein therectifier system is of the active type and the power turbine controllergenerates gate drive signals that change the load of the rectifiersystem on the electrical generator.
 5. The power turbine speed controlsystem of claim 1, further comprising an energy management system foradjusting the electrical load on the electrical generator.
 6. The powerturbine speed control system of claim 5, wherein the energy managementsystem comprises an energy storage system.
 7. The power turbine speedcontrol system of claim 5, further comprising an energy sink system. 8.The power turbine speed control system of claim 1, further comprising afuel system controller for sensing power consumed by the electrical loadand generating a fuel command signal that changes fuel delivered to thegas generator compressor spool in response to sensed changes in consumedpower.
 9. The power turbine speed control system of claim 8, furthercomprising an energy management system for adjusting the electrical loadon the electrical generator, wherein the fuel system controller sensesavailable energy from the energy management system and generates thefuel command signal responsive to both the available energy from theenergy management system and consumed by the electrical load.
 10. Thepower turbine speed control system of claim 8, wherein the fuel systemcontroller adjusts the fuel command signal responsive to a feed forwardsignal representative of predicted electrical transients in theelectrical load.
 11. The power turbine speed control system of claim 1,further comprising a fuel system controller for generating a fuelcommand signal that changes fuel delivered to the gas generatorcompressor spool in response to a feed forward signal representative ofpredicted electrical transients in the electrical load.